Solid state electrolytes have attracted interest for many years because of their wide applications in many types of solid state electrochemical devices. The solid state electrolyte, lithium phosphorous oxynitride (LiPON), in addition to having outstanding Li ion conductivity in solid state electrolytes, has excellent chemical and physical stability in general and, more importantly, at the interfaces with cathodes and anodes. However, due to poor ionic conductivities in LiPON, compared with liquid state electrolytes, the usages of solid state electrolytes are very much limited. There have been many attempts to improve ionic conductivity of solid state electrolytes by (1) optimizing deposition conditions and (2) changing the chemical composition of the solid. However, the improvements, so far, have yet to be significant—for example, the ionic conductivity for commonly used LiPON is still less than a few μS/cm.
In thin film batteries (TFBs) and electrochromic devices, a pinhole in the solid state electrolyte film can compromise the function of the device. For example, a pinhole in the solid state electrolyte film can reduce the breakdown voltage of the device, or worse still lead to a short between conducting layers and render the device useless.
FIG. 1 shows a cross-sectional representation of a typical thin film battery (TFB). The TFB device structure 100 with anode current collector 160 and cathode current collector 120 are formed on a substrate 110, followed by cathode 130, electrolyte 140 and anode 150; although the device may be fabricated with the cathode, electrolyte and anode in reverse order. Furthermore, the cathode current collector (CCC) and anode current collector (ACC) may be deposited separately. For example, the CCC may be deposited before the cathode and the ACC may be deposited after the electrolyte. The device may be covered by an encapsulation layer 170 to protect the environmentally sensitive layers from oxidizing agents. See, for example, N. J. Dudney, Materials Science and Engineering B 1 16, (2005) 245-249. Note that the component layers are not drawn to scale in the TFB device shown in FIG. 1.
In a typical TFB device structure, such as shown in FIG. 1, the electrolyte—a dielectric material such as Lithium Phosporous Oxynitride (LiPON)—is sandwiched between two electrodes—the anode and cathode. The conventional method used to deposit LiPON is physical vapor deposition (PVD) radio frequency (RF) sputtering of a Li3PO4 target in a N2 ambient. However, this deposition process can lead to a very significant yield loss due to pinholes in the LiPON films, and pinhole density increases with application of increasing RF power during sputtering. One approach to minimizing pinholes involves depositing thicker films of LiPON—typically one to two microns thick—and when the cathode has poor surface morphology the thickness of the LiPON may need to be greater yet. However, this is still not completely effective in removing pinholes and increases the cost of the process step due to lower throughput and more costly overhead in terms of consumed materials.
Similar considerations as for the TFB also apply to other electrochemical devices, such as the electrochromic device shown in FIG. 2.
Clearly, there is a need for improved solid state electrolyte films and deposition processes and equipment which can provide these solid state electrolyte films with higher ionic conductivity and lower pinhole density at low cost.